Bissextus
Bissext, or bissextus is the day which is added to the Gregorian calendar every fourth year to compensate for the six-hour difference in length between the common 365-day year and the actual length of the solar year.
Originally, the day was inserted after 24 February, i.e. the 6th day before the calends of March, Consequently, besides the sextus, or sixth before the calends, the bis-sextus or "second sixth," was 25 February. In modern usage, with the exception of ecclesiastical calendars, the intercalary day is added for convenience at the end of the month of February, as 29 February, and years in which February has 29 days are called "bissextile," or leap years.
